Acquired immune deficiency syndrome (AIDS) is a fatal disease, reported cases of which have increased dramatically within the past several years. Estimates of reported cases in the very near future also continue to rise dramatically. Consequently, there is a great need to develop drugs and vaccines to combat AIDS.
The AIDS virus was first identified in 1983. It has been known by several names and acronyms. It is the third known T-lymphocyte virus (HTLV-III), and it has the capacity to replicate within cells of the immune system, causing profound cell destruction. The AIDS virus is a retrovirus, a virus that uses reverse transcriptase during replication. This particular retrovirus is also known as lymphadenopathy-associated virus (LAV), AIDS-related virus (ARV) and, most recently, as human immunodeficiency virus (HIV). Two distinct families of HIV have been described to date, namely HIV-1 and HIV-2. The acronym HIV is used hereinafter to refer to HIV viruses generically.
Specifically, HIV is known to exert a profound cytopathic effect on CD4+ helper/inducer T-cells, thereby severely compromising the immune system. HIV infection also results in neurological deterioration and, ultimately, in the death of the infected individual.
The field of viral chemotherapeutics has developed in response to the need for agents effective against retroviruses, in particular HIV. Theoretically, there are many ways in which an agent can exhibit anti-retroviral activity. The HIV genome encodes several viral-specific enzymes, such as reverse transcriptase (RT), integrase and protease (PR); viral-specific regulatory proteins, such as tat, rev, nef and vif; and, numerous viral-specific structural proteins, and numerous viral-specific structural proteins, such as capsid, nucleocapsid, matrix, and envelope proteins. Many of these proteins are essential for viral replication. Accordingly, viral replication theoretically could be inhibited through inhibition of any one or all of the proteins involved in viral replication. In practice, however, only inhibitors of RT and PR are currently available for antiviral therapy.
Nucleoside analogues (NRTIs), such as 3′-azido-2′,3′-dideoxythymidine (AZT), 2′,3′-dideoxycytidine (ddC), and 2′,3′-dideoxyinosine (ddl) are known to inhibit HIV RT. There also exist non-nucleoside inhibitors (NNRTIs) specific for HIV-1 RT, such as nevirapine, and efavirenz.
Retroviral PR inhibitors (PIs) have also been identified as a class of anti-retroviral agents. The retroviral PR processes polyprotein precursors into viral structural proteins and replicative enzymes. This processing is essential for the assembly and maturation of fully infectious virions. Accordingly, the design of PIs that selectively inhibit PR has been an important therapeutic goal in the treatment of HIV infections and AIDS. Strategies used in the design of HIV PIs include substrate-based, peptidomimetic, transition state-based, and structure-based drug design (Wlodawer & Erickson, Ann. Rev. Biochem., 62, 543-585 (1992)).
Numerous classes of potent peptidic inhibitors of PR have been designed using the natural cleavage site of the precursor polyproteins as a starting point. These inhibitors typically are peptide substrate analogs in which the scissile P1-P1′ amide bond has been replaced by a non-hydrolyzable isostere with tetrahedral geometry (Moore et al., Perspect. Drug Dis. Design, 1, 85 (1993); Tomasselli et al., Int. J. Chem. Biotechnology, 6 (1991); Huff, J. Med. Chem., 34, 2305 (1991); Norbeck et al., Ann. Reports Med. Chem., 26, 141 (1991); Meek, J. Enzyme Inhibition, 6, 65 (1992)).
The design of HIV-1 PIs based on the transition-state mimetic concept has led to the generation of a variety of peptide derivatives highly active against viral replication in vitro (Erickson et al., Science; 249, 527-533 (1990); Kramer et al., Science, 231, 1580-1584 (1986); McQuade et al., Science, 247, 454-456 (1990); Meek et al., Nature (London), 343, 90-92 (1990); Roberts et al., Science, 248, 358-361 (1990)). These active agents contain a non-hydrolyzable, dipeptide isostere such as hydroxyethylene (McQuade et al., supra; Meek et al., Nature (London), 343, 90-92 (1990); Vacca et al., J. Med. Chem., 34, 1225-1228 (1991)) or hydroxyethylamine (Rich et al., J. Med. Chem., 33, 1285-1288 (1990); Roberts et al., Science, 248, 358-361 (1990)) as an active moiety which mimics the putative transition state of the aspartic protease-catalyzed reaction.
Two-fold (C2) symmetric inhibitors of HIV protease represent another class of potent HIV PIs which were created by Erickson et al. on the basis of the three-dimensional symmetry of the enzyme active site (Erickson et al., supra).
Typically, the usefulness of currently available HIV PIs in the treatment of AIDS has been limited by relatively short plasma half-life, poor oral bioavailability, and the technical difficulty of scale-up synthesis (Meek et al. (1992), supra). Although these inhibitors are effective in preventing the retroviral PR from functioning, the inhibitors suffer from some distinct disadvantages. Generally, peptidomimetics make poor drugs due to their potential adverse pharmacological properties, i.e., poor oral absorption, poor stability and rapid metabolism (Plattner et al., Drug Discovery Technologies, Clark et al., eds., Ellish Horwood, Chichester, England (1990)). Furthermore, since the active site of the PR is hindered, i.e., has reduced accessibility as compared to the remainder of the PR, the ability of the inhibitors to access and bind in the active site of the PR is impaired. Those inhibitors that do bind are generally poorly water-soluble, causing distinct problems for formulation and drug delivery.
There are currently six FDA-approved PIs for clinical use—Saquinavir, Ritonavir, Indinavir, Nelfinavir, Amprenavir and Lopinavir. When used alone or in combination with RT inhibitors, PIs dramatically suppress viral replication in HIV-infected individuals. Accordingly, PIs have become “first-line” antiviral agents for the control of HIV-1 (HIV) infections and are widely used in most highly active anti-retroviral therapy (HAART) regimens (Boden & Markowitz, Antimicrob. Agents Chemo., 42, 2775-2783, (1998)). Despite their success, the widespread use of PIs has led to the emergence of several thousands of genetically distinct, drug resistant HIV variants, many of which are cross-resistant to the PIs as a class (Richman, Adv. Exp. Med. Biol., 392, 383-395 (1996); Boden & Markowitz (1998), supra; Shafer et al. Ann. Intern. Med., 128, 906-911(1998)).
The ability of HAART to provide effective long-term antiretroviral therapy for HIV-1 infection has become a complex issue since 40 to 50% of those who initially achieve favorable viral suppression to undetectable levels experience treatment failure (Grabar et al., AIDS, 14, 141-149 (1999); Wit et al., J. Infect. Dis., 179, 790-798 (1999)). Moreover, 10 to 40% of antiviral therapy-naive individuals infected with HIV-1 have persistent viral replication (plasma HIV RNA>500 copies/ml) under HAART (Gulick et al., N. Engl. J. Med., 337, 734-739 (1997); Staszewski et al., N. Engl. J. Med., 341, 1865-1873 (1999)), possibly due to transmission of drug-resistant HIV-1 variants (Wainberg and Friedland, JAMA, 279, 1977-1983 (1998)). In addition, it is evident that with these anti-HIV drugs only partial immunologic reconstitution is attained in patients with advanced HIV-1 infection.
The clinical manifestations of drug resistance are viral RNA rebound and decreased CD4 cell-counts in the continued presence of drug. The majority of clinical resistance cases are due to viral adaptation through the generation and selection of mutations in the PR and RT genes. Mutant viruses can be generated through errors in reverse transcription of viral RNA, viral RNA synthesis, and recombination events (Coffin, Retroviruses pp.143-144, Cold Spring Harbor Laboratory Press, Plainview (1997)). Mutations within the protease gene that confer clinical drug resistance have emerged for all of the FDA-approved HIV PR inhibitors. The rapid development of drug resistance to PIs, combined with the transmissibility of drug-resistant HIV strains to newly-infected individuals, has resulted in the emergence of a new epidemic of multi-drug resistant AIDS (mdrAIDS). Multi-drug resistant AIDS is caused by a complex spectrum of genetically distinct, infectious new HIV strains that resist most or all forms of currently available treatment.
Accordingly, drug resistant HIV strains represent distinct infectious entities from a therapeutic viewpoint, and pose new challenges for drug design as well as drug treatment of existing infections. Substitutions have been documented in over 45 of the 99 amino acids of the HIV protease monomer in response to protease inhibitor treatment (Mellors, et al., International Antiviral News, 3, 8-13(1995); Eastman, et al., J. Virol., 72, 5154-5164(1998); Kozal, et al., Nat. Med., 2, 753-759(1996)). The particular sequence and pattern of mutations selected by PIs is believed to be somewhat drug-specific and often patient-specific, but high level resistance is typified by multiple mutations in the protease gene which give rise to cross-resistance to all of the PIs.
The challenge of tackling drug resistance is perhaps best illustrated by considering the dynamics of a typical HIV infection. Approximately 1012 virions are produced in an HIV infected individual every day. The mutation rate of HIV is approximately 1 per genome, which numbers 104 nucleotide bases. Therefore, every nucleotide in the genome is mutated 108 times per round of replication in the patient. This means that all possible single site mutations are present in at least the 0.01% level. Because of this, drugs that can be rendered ineffective with a single mutation from wild type have the shortest effective lifetime in monotherapy settings. The apparently large number of possible mutational pathways, possible mutational combinations, and the danger of generating class-specific cross resistance can make the choice of a subsequent protease inhibitor-containing combination regimen for “salvage therapy” seem very complicated and risky. Even the choice of protease inhibitor with which to initiate therapy, so-called “first-line” therapy, can be a risky enterprise that may inadvertently select for an undesired resistance pathway. Drug-naïve HIV-infected individuals pose even more of a risk for developing resistance to first-line therapies.
For the reasons outlined above, the development of new anti-HIV-1 therapeutics presents formidable challenges different from those in the design of the first line drugs, particularly in regard to consideration of selection pressure mechanisms in addition to the conventional issues of potency, pharmacology, safety, and mechanism of drug action. Indeed, HIV-1 can apparently develop resistance to any existing anti-HIV-1 therapeutic. In particular, the very features that contribute to the specificity and efficacy of RTIs and PIs provide the virus with a strategy to mount resistance (Erickson and Burt, Annu. Rev. Pharmacol. Toxicol., 36, 545-571 (1996); Mitsuya and Erickson, Textbook of AIDS Medicine, pp.751-780, Williams and Wilkins, Baltimore (1999)), and it seems highly likely that this resistance issue will remain problematic for years to come.
Despite numerous studies of drug resistance to PIs, successful strategies to design inhibitors directly targeted against drug resistant HIV have been lacking. Instead, efforts have been directed at identifying drugs with increased potency to wild type virus, and with longer pharmacological half-lives (exemplified by Amprenavir). Another approach has been to develop PIs that are sensitive to pharmacologic “boosting” using Ritonavir, a PI that is also a potent inhibitor of the cytochrome enzymes. The latter approach, exemplified by KALETRA (a lopinavir/ritonavir combination), involves higher total drug exposures to PIs which, over time, may lead to long term, serious side effects. Several other PIs have been identified based on efforts to improve plasma half-life and bioavailability. For example, PIs incorporating the 2,5-diamino-3,4-disubstituted-1,6-diphenylhexane isostere are described in Ghosh et. al., Bioorg. Med. Chem. Lett., 8, 687-690 (1998) and U.S. Pat. No. 5,728,718 (Randad et al.), both of which are incorporated herein by reference in their entirety. HIV PIs, which incorporate the hydroxyethylamine isostere, are described in U.S. Pat. No. 5,502,060 (Thompson et al.), U.S. Pat. No. 5,703,076 (Talley et al.), and U.S. Pat. No. 5,475,027 (Talley et al.).
Recent studies have revealed the structural and biochemical mechanisms by which mutations in the PR gene of HIV confer drug resistance in the presence of PIs. An important conclusion that emerges from the body of evidence on resistance to PIs is that HIV variants that exhibit cross-resistance to first-line PIs should be considered to be unique infectious agents. New therapeutic agents need to be developed to successfully treat patients infected with these viruses. New strategies for drug discovery need to be explored to develop effective protease inhibitor-based treatments for patients with multidrug resistant virus. HIV protease is one the most intensively studied molecular targets in the history of infectious disease.
More recently, new mutant strains of HIV have emerged that are resistant to multiple, structurally diverse, experimental and chemotherapeutic HIV PIs. Such mdrHIV strains are typically found in infected patients who have undergone treatment with a combination of PIs or with a series of different PIs. The number of reported cases of patients infected with mdrHIV is rising steadily. Tragically for these patients, the available options for AIDS chemotherapy and/or HIV management is severely limited or is, otherwise, completely nonexistent.
A biochemical fitness profiling strategy has recently been used to identify a novel subclass of potent PIs that have broad-based activity against mdrHIV (Gulnik et al., (1995) supra; Erickson et al., WO 99/67254; Erickson et al., WO 99/67417).
In view of the foregoing problems, there exists a need for inhibitors against drug resistant and mdrHIV strains. Further, there exists a need for inhibitors against drug resistant and multi-drug resistant HIV proteases (mdrPR). Further still, there exists a need for inhibitors of HIV that can prevent or slow the emergence of drug resistant and mdrHIV strains in infected individuals. Inhibitors with the ability to inhibit mdrHIV strains, and to slow the emergence of drug resistant strains in wild type HIV infections, are defined as “resistance-repellent” inhibitors. There also exists a need for robust methods that can be used to design “resistance-repellent” inhibitors.